The present invention relates to a developing unit for use in an image forming apparatus of electrophotographic copying machines, laser beam printers and the like, and more particularly to a toner replenishing mechanism in a developing unit of image forming apparatus capable of selectively actuating a plurality of the developing units in accordance with an optionally predetermined area set in the direction of action for a single image forming operation.
The present inventor has proposed in his previous Japanese Patent Application No. 21630/1987 an image forming apparatus which is able to form an image having different colors corresponding to an area by selectively actuating a plurality of developing units in accordance with an optionally predetermined area set in the direction of action for a single image forming operation.
In the Japanese Published Examined Patent Application No. 2662/1987, there is disclosed a developing unit which is designed to continue actuating toner replenishing means and stirring means until the toner density reaches a standard value even if copying operation is finished if the toner density is lower than that of the standard value.
In an image forming mode by the image forming apparatus previously disclosed by the present inventor, in which development is made for a predetermined area set by the first developing unit and the rest of the area by the second developing unit during a single operation of image formation, when a toner density short signal is emitted during the developing process by the first developing unit, the toner replenishing motor is actuated, and when developing process is changed from the first developing unit to the second developing unit while toner replenishment occurs, the toner replenishing motor is instantly stopped corresponding with the stoppage of the first developing unit. The invention was developed so as to deal with the problem in which toner replenishing is continued when the first developing unit stopped its action and the replenished toner accumulated at the toner receiving inlet of the developing unit and finally overflows.
Because the first developing unit can not be actuated when the copying machine is changed to the second developing unit, toner replenishing action can no longer be continued as disclosed in Japanese Published Examined Patent Application No. 2662/1987. Accordingly, if the toner replenishment is limited to the time the first developing unit is in action, the toner consumption may exceed the the amount of toner replenished, and it causes a shortage of toner in the developing unit and eventually lowers image density because no function is provided against such problem.